Pulse-reflection distance measuring system



y 6, 1950 H. o. WRIGHT 2,57,74

PULSE-REFLECTION DISTANCE MEASURING SYSTEM Filed May 1, 1942 MIR/ABLEcows/mm s ur sum/a, CAM coMMumroR AND SWITCH /-'/G I'IEZ' sq ,as as f ron: 52%; INDICATOR 22 act TRAMS- REC. 8 HITTER 1,: 4H

FIG. 2 REC'D REFLECTED PULSE 40 42 40 ,2 4/ J. flu TRANSMITTED PULSE FREOUE NC V INVE/WDR h. 0. WRIGHT ATTORNEY Patented May 16, 1950 UNITEDTES PATENT OFFHE PULSE-REFLECTION DISTANCE MEASURING SYSTEM Henry 0.Wright, Suflem, N. Y., assignmto Bell Telephone LaboratoriesIncorporated, New

6 Claims.

This invention relates to improvements in the art of measuring distanceby the pulse-reflection method.

In a preferred form the principles of the invention are embodied in anelectromagnetic wave pulse-reflection type distance measuring system inwhich relatively short pulses are transmitted from an observation pointto impinge upon a surface, the distance of which is to be measured;reflections of these pulses are received at the observation point andthe time of travel of the pulses is determined to obtain the distancefrom the observation point to the reflecting surface. The requiredtiming is effected by combining the received reflected pulse with aheterodyne frequency which is caused to vary unidirectionally andcontinuously over a predetermined frequency range during a time intervalexceeding the time corresponding to that required for a pulse to bereflected from a surface at the maximum distance which it is desired tomeasure and determining the frequency of the resulting beat-note pulsesto obtain an indication of the distance to the reilecting object. Theheterodyne oscillator frequency is varied unidirectionally in a regularmanner, preferably in accordance with a substantially straight line,sinusoidal or other convenient law of variation, during the aforesaidinterval, starting as a pulse is transmitted, at the end of whichinterval it repeats its frequency variation cycle starting as a secondpulse is transmitted, the cycle of events just described recurringregularly so that a series of beat-note pulses are obtained as willbecome apparent hereinafter.

Objects of the invention are to provide improved methods of andapparatus for measuring distance.

A further object is to provide an improved method of effecting thetiming of electromagnetic wave pulses in a pulse-reflection type radioenergy-wave distance measuring system.

Another object is to provide a distance measuring system combining theadvantages of the pulsereflection and the frequency-modulated energywavetypes of distance measuring systems and to avoid a number of thedimculties encountered in The principles of the invention will be more 2readily understood in connection with the following description of aparticular preferred embodiment thereof illustrated by the accompanyingdrawing in which:

Fig. 1 shows in block schematic diagram form a radio-wave distancemeasuring system of the invention;

Fig. 2 illustrates the sequence of events throughout taro operationalcycles of the system of Fig.

i an

Figs. 3 to 5, inclusive, show structural details of a variable condenserand cam-operated switch for suitably controlling the frequencymodulation 1of the heterodyne oscillator of the system of In moredetail, in Fig. 1 a motor [6 carries on the left end of its shaft acommutator I? which closes the circuit between brushes It for a briefinterval twice during each revolution of the shaft and causes radiotransmitter Hi to energize antenna 28 to emit a short pulse of energytoward a reflecting surface 3! On the right end of the shaft of motor I5is mounted a rotary variable capacitance l8 which varies continuouslyfrom minimum to maximum capacity twice during each revolution of themotor, each capacity variation cycle commencing with the closure of thecircuit of brushes I 6 and ending just prior to the next closure of thiscircuit. A suitable variable capacitance is illustrated in Figs. 3 to 5,inclusive, and described below.

Variable condenser I8 is connected electrically in thefrequency-determining circuit of heterodyne oscillator 29.

Wave 38 represents transmitted pulses emitted from antenna 28 and wave32 represents reflec tions thereof returning to receiving antenna 2%which is connected to receiver 22 in which they can be detected andamplified. The outputs of receiver 22 and heterodyne oscillator 20 areintroduced into detector amplifier 34 where they are combined to producea beat-note frequency which is amplified and introduced into afrequency-measuring circuit 36 and the output of which is supplied to anindicator 38. The latter two devices can be of the types described indetail inPatent 2,247,662 issued July 1, 1941, to R. C. Newhouse anddesignated by numerals 23 and 24 of Fig. 1 of said patent.

A shield 26, as shown in Fig. 1 of this application, is preferablyinterposed between the transmitting and receiving antennas to preventthe direct reception of the transmitted pulses, unless the antennas aresufliciently directive to prevent such direct transmission without sucha shield. Reflectors 20 and 23 increase the directive characteristics ofthe respective antennas with which they are associated. A reasonablybroad directive characteristic is desirable however to avoid thenecessity of pointing either antenna to insure that reflections fromobjects within the intended range of operation will be received byantenna 24. In

general radio waves of wave-lengths between 1 centimeter and 1 meter arepreferable for use in altimeter and object-locating systems.

Referring to Fig. 2, it is apparent that the beat- .note frequencieswhich result from the combination of the reflected pulses 42 with theheterodyne oscillator output, curve 44, will have a mean frequencydependent upon the time interval between the transmission of a pulse andthe receipt of its ,reflection. The output of the frequency-measuringcircuit I6 will in turn be proportional to the mean frequency of thebeat-notes received and the indicator 38 can therefore be calibrated toread distance directly. As mentioned above it is preferable that thetransmitted pulses 40 should not reach antenna 24 so that the indicator.38 will normally produce no indication unless reflected pulses are beingreceived. Of course a zero setting of the indicator could be made tocompensate for the effects of directly received pulses if it is deemeddesirable to omit shield 26 and reflectors 23 and 29.

In Figs. 3 and 4 the mechanical structure of a variable capacitancehaving a single set of rotor plates 52 covering an angle of slightlyless than 180 degrees and two sets of stator plates 50 and 54 opposed toeach other and each group of stator plates also covering an angle ofSlightly less than 180 degrees is shown. A cam 56 operating a switch 58,60 and 62, Fig. 5, serves to alternately connect to one or the other ofthe two sets of stator plates 5|! and 54 so that the effective capacityof the condenser is instantaneously shifted from maximum to minimum atthe proper moment to begin the next successive cyclic. capacity change.The cam 56 and set of rotor plates 52 are mounted on shaft 64 at theright end of motor l6. Fig. 5 shows the electrical circuit of thevariable capacitance and cam-operated switch associated therewith,terminals 59 being connected to the heterodyne oscillator 20. v

Systems of the invention have the distinct advantage that thetransmitted wave pulses are of substantially constant frequency and yetthe advantage of timing by frequency modulation is retained. The designof the transmitter, receiver and their associated antennas, forsatisfactory performance is thereby greatly simplified.

Also receiver 22 can preferably as above men-' tioned include a fixedfrequency heterodyne oscillator and intermediate frequency amplificationso that considerable latitude in the choice of the location in thefrequency spectrum of the frequency range over which heterodyneoscillator 20 can conveniently be varied is readily afforded. Since theoutput power of the heterodyne osci1- lator need be only a smallfraction of the transmitter output, its design is a relatively simplematter, as compared with that of a high powerfrequency-modulatedtransmitter.

Obviously compressional wave systems employing the principles of theinvention can readily be constructed and systems embodying theprinciples of the invention can be employed in timing regularlyrecurring phenomena in other than distance measuring systems, forexample, in the timing of "echoes or reflections in long metalliccircuits, wave guides and the like. The scope of the invention istherefore, obviously, not limited to the preferred illustrativeembodiment above described.

For example, if it is deemed desirable, the pulses transmitted canreadily be made to vary in frequency at the same rate as frequency ofthe heterodyne oscillator so that the beat-note frequency pulses will beof uniform frequency throughout each cheat-note pulse and numerous othermodifications within the scope and spirit of the invention may readilybe made by those skilled in the art. No attempt has here been made toexhaustively cover such modifications. The scope of the invention isdefined in the followingclaims.

What'is claimed is:

1. A radio distance measuring system comprising a transmitter ofsubstantially constant pure carrier frequency radio-wave energy, meansfor actuating said transmitter to .emit short pulses of radio-waveenergy at regularly recurrent time intervals, said intervals being atleast of sufficient lengthto permit a transmitted pulse to travel to areflecting surface at the maximum distance to be measured and to returnby reflection to the system before the next successive pulse is emitted,means separate from and having no eifect upon the output ofsaid-transmitter, for producing electrical wave energy of a heterodynefrequency continuously varying over a predetermined range of frequenciesduring each said time interval, means for receiving reflections of saidtransmitted pulses from a surface the distance of which is to bemeasured, means for combining said received reflected pulses and saidheterodyne frequency energy and detecting the beatnote frequenciestherebetween, meansfor determining -the frequencies of said beat-notesto obtain an indication of the distance of said reflecting surface.

2. In a wave-energy pulse-reflection distance measuring system of thetype in which energy pulses of substantially constant pure carrierfrequency are emitted at predetermined intervals, said intervals beingat least of suflicient length to permit a transmitted pulse to travel toa reflecting surface at the maximum distance to be measured and toreturn by reflection to the system before the next successive pulse isemitted, reflections of said pulses from-a remote surface are receivedand the distance to the remote surface is determined by measuring thetime interval between the emission of a pulse and the receipt of itsreflection, the method of measuring the reflection time interval whichcomprises frequency-modulating a separate auxiliary source of heterodynefrequency wave-energy continuously, and unidirectionally over apredetermined range of frequencies during the intervals between theemission of successive energy pulses, said separate source having hoeffect upon the said emitted energy pulses, combining the reflectedpulses with energy from said auxiliary source to obtain beat-note pulsesand determining the frequency of said beat-note pulses to obtainindications of the reflection time interval of the reflected pulses.

' 3. A wave-energy distance measuring system of the pulse-reflectiontype comprising means for emitting wave-energy pulses of substantiallyconstant pure carrier frequency at regular intervals exceeding -thereflection time of said pulses from the greatest distance it is desiredto measure, means for receiving reflections of said pulses from remotesurfaces, a separate auxiliary source of heterodyne wave energy, havingno efiect upon the output of said emitting means, means for continuouslyand unidirectionally frequency modulating said auxiliary source over apredetermined frequency range during each interval between successiveemitted pulses, means for combining the received reflected pulses withthe energy from said auxiliary source to obtain beat-note frequenciesand means for determining said beatnote frequencies to obtain anindication of the distance of the surface from which reflected pulsesare being received.

4. In a radio distance measuring system, a radio transmitter, adirective antenna cooperatively associated therewith, a radio receiver,a second directive antenna cooperatively associated therewith, a keyingmeans and motive means therefor cooperatively associated with saidtransmitting apparatus to cause the latter to emit short energy pulsesof substantially constant pure carrier frequency at regular intervals,said intervals being at least of suflicient length to permit atransmitted pulse to travel to a reflecting surface at the maximumdistance to be measured and to return by reflection to the system beforethe next successive pulse is emitted, a heterodyne oscillator, avariable impedance element connected in the frequency-determiningcircuit thereof, motive means cooperatively associated with saidvariable impedance to vary it through a predetermined range of valuesduring the interval between successive pulses of said transmitter,detecting means electrically connecting to the outputs of said receiverand said heterodyne oscillator, a frequency-measuring circuitelectrically connected to the output of said detecting means and anindicator electrically connected to the output of said last-statedcircuit whereby when pulses of energy are directed by said transmittingantenna toward a remote object and reflections of said pulses arereceived from said object by said receiving antenna, an indication -ofthe distance to said object will be obtained.

5. A radio object-detection and distance measuring system comprising atransmitter having a substantially constant pure carrier frequency, akeying device cooperatively connected with said transmitter andperiodically operative to cause said-transmitter to generate a shortenergy pulse, the periodicity of said keying device being such that thetime interval between successive pulses is at least sufficient for apulse to travel to a. reflecting object at the maximum distance to bemeasured and to return by reflection to the system before the nextsuccessive pulse is generated, an antenna connected to said transmitterto radiate the generated energy pulses. an antenna adapted to receivereflections of the radiated pulses, a receiver and intermediatefrequency amplifier cooperatively coupled with said receiving antenna, aheterodyne oscillator having an impedanceelectrically connected to itsfrequency determining circuit, said impedance being continuouslyvariable throughout a predetermined range, a mechanism cooperativelycoupled with said variable impedance and said keying device to vary saidimpedance through said range during each time interval elapsing betweenthe keying of successive pulses, a detector the input of which iselectrically connected to the output circuits of said receiver-amplifierand said heterodyne oscillator, a frequency measuring circuitelectrically connected to the output of said detector and an indicatorelectrically connected to the output of said last stated circuit.

6. A pulse-reflection timing system comprising an energy pulse emitter,said emitter emitting short pulses of substantially constant pure car=rier frequency at intervals at least sufflciently long to permit eachpulse to travel to a reflecting surface at the maximum distance to bemeasured and to return by reflection to the system before the nextsuccessive pulse is emitted, a. receiver of reflections of the emittedpulses, a separate source of variable frequency heterodyne energy saidsource having no effect upon the output of said emitter, said sourcebeing operatively coupled with said emitter to initiate a cycle ofcontinuous frequency variation over a period exceeding the greatestreflection time to be measured, a combining and demodulating deviceoperatively coupled to said receiver and said variable frequency sourceproviding energy beat notes representing the instantaneous frequencydifference between the received reflected pulse and the source ofvariable frequency, a frequency measuring device operatively connectedto said combining and demodulating device and an indicating deviceoperatively connected to said frequency measuring device.

. HENRY O. WRIGHT.

REFERENCES CITED The following references are of record in the flle ofthis patent:

UNITED STATES PATENTS Number Name Date 2,011,392 Bentley Aug. 13, 19352,423,644 Evans July 8, 1947 FOREIGN PATENTS Number Country Date 528,345Great Britain Oct. 28, 1940

